Conventionally, room-temperature bonding methods are performed in a vacuum as described in Patent Document 1 or in inert gas as described in Patent Document 2 after a bonding surface is cleaned using an energy wave. This is because, if the bonding surface is exposed to the atmospheric air, oxidation or readhesion of organic substances or the like prevents metal atoms from being attracted by each other.
In the case of Patent Document 2, if gold is used, which does not react, oxidation is prevented, but adhesion of organic substances remains. Therefore, bonding is performed in inert gas.
The bonding surface is polished to a profile irregularity of the order of several nanometers before being subjected to bonding under a low load. This is because the profile irregularity of the order of several nanometers can lead to spontaneous bonding due to intermolecular force in a vacuum.    Patent Document 1: JP No. 2791429 B    Patent Document 2; JP No. 2001-351892 A